1. Field of the Invention
The present invention relates generally to DC current sources. More particularly, the present invention relates to circuits which generate high impedance direct currents from commonly available alternating current type signals, where the direct currents generated are used in telecommunications applications.
2. State of the Art
Historically, many applications have existed in the telephone local loop environment where it is desirable to generate power from the central office (CO) which will power remote station equipment or repeaters over the same wires used for communications. A conventional telephone is a common application where a 48 V station battery is used to power remote site telephones. Other applications include the transmission of power which is necessary in conjunction with standard T1 loops, and power used in generating a "sealing current" in basic rate two-wire ISDN and the four-wire DDS services for purposes of maintaining splice integrity (i.e., sealing), indicating loop continuity, and signaling.
With regard to T1 loops, because several watts need to be generated, and because it is impractical using the 48 V station battery to generate and send over reasonable distances more than one watt using standard techniques, the T1 systems typically use higher voltage batteries (e.g., 120 V). Such prior art systems use two pairs of wires in which the power is conveyed by a phantom circuit formed by the two two-wire pairs. The use of a 120 V battery to power a loop system, although simple and direct conceptually, poses a number of difficulties and problems. For example, there is no DC isolation between the individual circuits and earth. In addition, because 60 V is considered as a maximum safe voltage, the 120 V battery presents human harms issues when present. Further, 120 V batteries are not always available, and even when available are not particularly efficient.
With regard to the sealing currents, the sealing current is sourced at the central office and terminated at the customer premises equipment (CPE). Typically, the amperage of the sealing current is defined within a specific range which must be independent of whether or not the loop is shorted or is of maximum length. Because the range must be independent of loop conditions, a source with a relative high impedance, i.e., a current source, is required. Conventionally, in order to provide such a current source, the 48 V (-48 V) central office station battery with a series resistance is utilized in a circuit as shown in prior art FIG. 1. In particular, in prior art FIG. 1, the central office 11 is shown with a transmitter 13, a transmitting transformer 15, a receiver 17, a receiving transformer 19, and first and second resistors 21 and 23. Resistor 21 is coupled between ground and the center tap of the local loop side of the transmitting transformer 15, while resistor 23 is coupled between a -48 V voltage source 24 and the center tap of the local loop side of the receiving transformer 19. The local loop side of the transformers 15 and 19 are shown coupled to the wires 25a, 25b, 25c, 25d of the local loop 25 in a four-wire DDS arrangement. In turn, at the CPE, a data service unit (DSU) 27 is shown with a receiving transformer 29, a receiver 31, a transmitting transformer 33, a transmitter 35, and a termination circuit 37. The receiving transformer 29 of the DSU 27 has one winding coupled to wires 25a and 25b, and its center tap coupled to the termination circuit 37 which provides a load resistance, while the transmitting transformer 33 of the DSU has one winding coupled to wires 25c and 25d, and its center tap coupled to the termination circuit 37.
The circuit shown in simplified format in FIG. 1 generates a sealing current which runs through wires 25a and 25b to the termination circuit 37, and back through wires 25c and 25d. In particular, and as shown in the equivalent circuit of FIG. 2, with a -48 V .+-.7 V voltage source 24 (which is normally between 43 V and 53 V), and with a load resistance R.sub.e of between 0 and 2500.OMEGA. (normally between 1300.OMEGA. and 2000.OMEGA.) at the termination circuit 37, if it desirable to generate a sealing current between 4 mA and 20 mA, the resistors 21 and 23 must be chosen carefully. In particular, it can be shown that the total resistance R.sub.g of resistors 21 and 23 should be at least 2750.OMEGA., and no more than 7750.OMEGA.. Thus, in one extreme situation, where the source voltage is -55 V, and the load resistance is shorted (i.e., R.sub.e =0), in order for the maximum sealing current to be 20 mA, the resistance R.sub.g must be equal to at least 2750.OMEGA.=55 V/20 mA. In another extreme situation, where the source voltage is -41 V, and the load resistance is 2500.OMEGA., in order for the minimum sealing current to be 4 mA, the resistance R.sub.g must be equal to at most 7750.OMEGA., as 7750.OMEGA.2500.OMEGA.=41 V/4 mA.
If resistor R.sub.g is chosen to be a 6000.OMEGA. resistor, then with a -48 V source and a short in the load, the power dissipated in the resistor R.sub.g would be 0.384 W. With the same resistor and same source, where the load is the typical 2000.OMEGA., the power dissipated in the resistor R.sub.g would be 0.216 W.
While the sealing current source circuitry of the prior art utilizing the 48 V battery voltage source is effective in providing the desired sealing currents, it is not ideal for several reasons. First, as suggested above, the 48 V source in conjunction with the large resistor R.sub.g undesirably dissipates considerable power. Second, with circuitry using a common 48 V source, connections to ground are required, raising issues of compliance with network regulations, and harms. Third, 48 V sources are not always readily available; and even where available may not be cost effective.